The present invention relates to a display apparatus such as a plasma display panel (PDP) and a digital micro-mirror device (DMD), and more particularly, to a display apparatus which provides a half-tone display using a time division of a picture signal of one field into a plurality of sub-fields.
In a plasma display device or the like, a display output exhibits a non-linear response, in particular, a saturation response with respect to the magnitude of an input voltage, and accordingly, a half-tone can not be correctly displayed if it is attempted to provide a half-tone display by an amplitude modulation of the picture signal. For this reason, a half-tone display is currently provided by dividing a time of one field into a plurality of sub-fields, and setting relative ratio of luminescent time in respective sub-fields at 1:2:4:8: . . . (which are n-th power of 2, where n is an integer), for example, so that a combination of luminescence and non-luminescence in the respective sub-fields for each picture element is changed to establish a gradation level represented by a total sum of luminescent time in one field.
FIG. 9 is an illustration of luminescence sequence of a conventional display apparatus. In FIG. 9. each of hatched portions represents a discharge sustained period, which is immediately preceded by an address period which is a portion indicated by X. FIG. 9 shows an example in which one field is divided into eight sub-fields SF8-SF1. The relative ratio of luminescent time of individual sub-fields SF8, SF7, SF6, SF5, SF4, SF3, SF2 and SF1 is 128:64:32:16:8:4:2:1. A capability to display 256 gradations is obtained by a combination of luminescence and non-luminescence in these sub-fields SF8-SF1.
For example, when a display is to be provided at a gradation "127", luminescence takes place in the sub-fields SF7, SF6, SF5, SF4, SF3, SF2 and SF1, while non-luminescence is chosen for the sub-field SF8. A human eye has a time integrating effect, and can not respond to on/off of luminescence which takes place in one field. Accordingly, luminescence from the sub-fields SF7, SF6, SF5, SF4, SF3, SF2 and SF1 is integrated and then perceived by the human eye as if display is provided at the gradation "127".
When performing a digital signal processing of a picture signal, the signal is quantized using from 6 to 10 bits or greater bits depending on the intended purpose. A quantization using 8 bits will be described here. However, it is to be understood that when the number of bits used in quantization is changed, there results a change in the number of the sub-fields which are divided, but that there results no essential change in the fundamental operation.
When a picture is to be displayed by the display apparatus, the picture signal is initially converted into an 8 bit digital signal, and the most significant bit (bit 8) is allocated to the sub-field SF8, and the next most significant bit (bit 7) is allocated to the sub-field SF7. Similarly, the less significant bits 6, 5, 4, 3, 2 and 1 are allocated to the sub-fields SF6, SF5, SF4, SF3, SF2 and SF1, respectively.
FIG. 10 is a block diagram showing an arrangement for the conventional display apparatus. As shown in FIG. 10, the conventional display apparatus has an input terminal 1 to which a picture signal is input, an input terminal 2 to which a sync signal is input, an A/D converter 3 in which the picture signal input to the input terminal 1 is converted into a digital signal, a frame memory 5 which stores two frames of the output signal from the A/D converter 3, a driver 6, a display 7 such as the plasma display panel, and a controller 8. The controller 8 controls the A/D converter 3, the frame memory 5 and the driver 6 on the basis of the input sync signal. The driver 6 drives the display 7 on the basis of the output signals from the frame memory 5 and the controller 8.
The operation of the display apparatus shown in FIG. 10 will now be described. The controller 8 delivers given control signals to the A/D converter 3, the frame memory 5, and the driver 6 on the basis of the sync signal which is input to the input terminal 2. The picture signal which is input to the input terminal 1 is converted to eight bit digital data in the A/D converter 3 and is stored in a given location within the frame memory 5. It is to be noted that the frame memory 5 includes a first frame memory section and a second frame memory section, and the input data is alternately written into the first frame memory section and the second frame memory section.
First, in response to a command from the controller 8, data stored in the frame memory 5 is read out therefrom, specifically, bit 8 being read out during the address period for the sub-field SF8 shown in FIG. 9. It is to be understood that data is read out from the memory section of the frame memory 5 to which a write operation is not being made. Data read out is fed through the driver 6 to be delivered to the display 7. When the display 7 is the plasma display panel of AC type, the panel has a memory effect which allows written data to be maintained during a period of time required for data for the whole screen to be written into the display 7. The display 7 into which given data is written is activated by the driver 6 to cause luminescence from picture elements during the discharge sustained period of the sub-field SF8.
During the address period for the next sub-field SF7, bit 7 is read out from the frame memory 5 and fed through the driver 6 to be delivered to the display 7 which causes luminescence during the discharge sustained period of the sub-field SF7 in the similar manner mentioned above in connection with the sub-field SF8.
Subsequently, bits 6, 5, 4, 3, 2 and 1 for the sub-fields SF6, SF5, SF4, SF3, SF2 and SF1, respectively, are read out from the frame memory 5 during the address periods of the respective sub-fields, and fed through the driver 6 to be delivered to the display 7. Luminescence from the picture element corresponding to the data which are read out form the frame memory 5 takes place during the discharge sustained periods of the respective sub-fields SF6, SF5, SF4, SF3, SF2 and SF1.
With the conventional display apparatus constructed in the manner mentioned above, it occurs that when an image which varies smoothly in the horizontal direction moves horizontally across the screen, a vertical strip-shaped band, which was invisible when the image was at rest, appears to be perceived, such band being hereafter referred to as "false profile". The false profile is a dark or colored band. The band becomes colored when certain one of primary color components R, G and B is reduced. This phenomenon will be further described with reference to FIG. 11 and FIG. 12.
FIG. 11 is an illustration of an up-shift of gradation in an image which occurs in the conventional display apparatus. In FIG. 11, the abscissa represents a horizontal direction of the screen, while the ordinate represents a time. FIG. 11 shows six picture elements which follow one after another in the horizontal direction. In FIG. 11, an image in which the gradation smoothly varies in the horizontal direction is shown on the six picture elements which follow in the horizontal direction to produce an up-shift to the most significant bit. More specifically, FIG. 11 shows an image in which the gradation changes from "127" to "128" between the 2nd and the 3rd picture elements as counted from the left.
FIG. 12 is an illustration which explains the phenomenon of the false profile occurring in the conventional display apparatus. FIG. 12 shows three images when the image shown in FIG. 11 is shifted by one picture element to the right for every field. Thus, FIG. 11 corresponds to the uppermost field shown in FIG. 12. In both FIG. 11 and FIG. 12, the picture elements, which are used for display to represent a gradation "127", effect luminescence during the sub-fields SF7, SF6, SF5, SF4, SF3, SF2 and SF1, while the picture elements, which are used for display to represent a gradation "128", effect luminescence only during the sub-field SF8. Considering the central picture elements as viewed in the horizontal direction of FIG. 12 (or the 3rd and the 4th picture elements as counted from the left side), it will be noted that the gradation changes from the "128" to "127" as time passes, producing a down-shift from the sub-field SF8 to less significant sub-fields SF7, SF6, SF5, SF4, SF3, SF2 and SF1.
In FIG. 12, broken lines A, B, C and D are conceptual lines of vision. When viewing a still image, the lines of vision will be directed vertically and no false profile will be produced. By contrast, when viewing a moving picture, the lines of vision follow a moving image, and accordingly, the lines which conceptually represent the lines of vision will run askew as indicated by the broken lines A, B, C and D in FIG. 12. A repetition of luminescence and non-luminescence (or a combination of on/off) which occurs within one field shown in FIG. 12 takes place in a short period of time, and accordingly, a time integrated value is provided for the perception of the brightness. Accordingly, integrating the broken lines A, B, C and D shown in FIG. 12 with respect to the time provides a relative perception value as indicated at the bottom of FIG. 12, and it will be noted that a reduction in perception value will be noted between the broken lines B and C. Stated differently, the gradation "127" is perceived on the retina corresponding to an area from the broken line A to B, and gradation "128" will be perceived in a region of retina which corresponds to an area between the broken lines C and D. However, in a region of retina which corresponds to an area between the broken lines B and C, there occurs a reduction in the perception value, a minimum value of which becomes equal to substantially zero. This reduction in perception value is recognized as the false profile.
This phenomenon is perceivable when an image having a change in the gradation from gradation "128" in which the luminescence occurs only during the sub-filed SF8 to a gradation "127" in which the luminescence occurs during the sub-fields SF7, SF6, SF5, SF4, SF3, SF2 and SF1 or an image in which a down-shift from a more significant bit to a less significant bit, or conversely an up-shift from a less significant bit to a more significant bit occurs across the screen. False profile is perceived, not only during an up-shift to or a down-shift from the most significant bit, but also during an up-shift to or a down-shift from a relatively high significant bit, for example, an up-shift or a down-shift occurring between the sub-fields SF7 and SF6.
As described above, when an image, which smoothly varies and includes a down-shift from a bit of relatively high significance to a bit of relatively low significance or includes an up-shift, moves horizontally across the screen in the conventional display apparatus, a false profile, which was invisible when the image was at rest, becomes perceived.